The entertainment, theater and convention business requires repeated set up and tear down of lighting instruments in various configurations. Lighting instruments are typically supported on horizontal pipe battens, vertical boom pipes, scaffolding or other elongate structural members. The lighting instrument includes a yolk which is bolted to a clamp. The clamp, which is most commonly a C-clamp, is then clamped onto the structural member by tightening an adjustment bolt provided on the clamp. Lighting instruments are usually remounted to different support structures between various theatrical productions to obtain the preferred lighting of the scene. Each clamp typically remains secured to its lighting instrument, so that a light can be removed from one fixed location and set at a new location by releasing the clamp from one support structure and reaffixing the clamp to a different support structure or to a different position along the same support structure. Conventionally, side arms, hardware structures composed of a C-clamp, a tubular member and a sliding "T" that can be placed along the tubular member, may be added between the lighting instrument and the clamp when necessary to have the lighting instrument offset horizontally from the structural member for proper focus. In some cases, lighting instruments need to be mounted on formed tracks rather than pipes, requiring the use of alternate clamping arrangements.
When unclamping a lighting instrument from a structural member, it is necessary to back out the adjustment bolt sufficiently to allow disengagement from the structural member. For reinstallation of the clamp, the adjustment bolt must then be rethreaded inwardly for fastening to another structural member, which may have a different diameter than the prior structural member with which the clamp was used necessitating more time for reinstallation. The backing out and rethreading of the adjustment bolt is labor intensive and time consuming. Often in attempts to make such adjustments and to ensure a firm grip of the clamp on the structural member, adjustment bolts may be over-torqued by a technician bending the adjustment bolt. This either ends the useful life of the clamp or poses a potential structural weakness. A further problem of conventional clamps is that the body is most often formed from cast iron, which may crack after repeated overstressing.
An example of a conventional C-clamp used for mounting lighting instrument is disclosed in U.S. Pat. No. 1,754,929 to Atticks. This type of clamp is commercially available from Altman Corporation. Conventional clamps include an aperture formed in the base of the clamp into which a spigot is inserted for mounting the lighting instrument to the clamp. The spigot has an enlarged diameter head that will not pass through the aperture and a stem projecting from the aperture. The spigot is antirotatably secured within the aperture by tightening a radial set bolt carried in the base of the clamp against the stem of the spigot. The yolk of the lighting instrument is bolted to the projecting end of the spigot by threading a bolt into the end of the spigot. To change the lighting instrument to another mounting means, it is necessary to back out the bolt and attach another mounting means to the yoke by this same bolt. Again, this adjustment is time consuming and labor intensive.
One conventional attempt to provide a clamp that may be more rapidly clamped and unclamped on structural members is disclosed in U.S. Pat. No. 3,784,140 to Auerbach. This clamp is sold commercially under the trademark SURECLAMP.TM.. The clamp includes a pivoting clamping jaw which is actuated by turning a threaded knurl knob to loosen a locking mechanism, which enables the rod to be moved longitudinally to pivot the clamp open and closed. While this clamp does provide for quicker adjustment, design factors of the SURECLAMP.TM. limit its use to light-weight fixtures and the fixture position is limited to being directly beneath the horizontal structural support.
An additional limitation of conventional clamps is the lack of a provision to attach a lift line to the clamp to pull the lighting instrument to a distant location which may be at a high elevation. Conventionally this is done by tying a line to the instrument or clamp, which is again time consuming. Additionally, once the clamp is secured in place by tightening the adjustment bolt against the structural support it is sometimes desirable to secure a safety cable between the pipe and instrument. Conventionally this is done by threading the safety cable through the yolk of the instrument and around the pipe or structural support, followed by tying or clipping the safety cable. Again, affixing safety cable is time consuming.